Sorry but you're conflating a lot.
This was about diabetes, not obesity. While there is overlap and they cover that, this was specifically about an intervention to reverse diabetes.
Second this is an intervention, aka a specific dietary program for diabetes. To compare that to worldwide obesity is comparing two extraordinarily different things. Worldwide obesity is likely from unhealthy processed foods that people choose to eat. This is an intervention to fix that. You can't compare those to shoo away the medical knowledge we have about interventions/programs.
So on to interventions/programs: Our knowledge of pretty much everything is vastly better than it was before. We understand macronutrients better, micronutrients better (vitamins, minerals), the entire biochemistry of the body - but especially about diabetes. Not only did we not know this 40 years ago, the ad-driven private industry didn't care about it.
Private industry wants to sell a product, they don't care about anything else. Like if you think private industry actually cared about solving diabetes 40 years ago, I got news for you: They don't give two shits. All they want to do is sell stuff. They can do the jazz hands but then they will wipe their hands clean with 'part of blah blah blah'. They truly do not care one iota.
NHS is a government agency that is actually trying to solve a problem. They are tasked to solve the problem. They want to solve the problem. Dare say, they need to solve the problem. NHS doesn't care about profits or market share. They care about solving the problem. Entirely different motivations, goals, structures, and definition of success.
And we do have the results:
Research on the effectiveness of the diet to manage diabetes initially began with promising results from randomised control trials. The diet was then rolled out to larger scale trials in clinical practice, implemented via the NHS.
The results confirmed that total diet replacement plans can induce weight loss as well as a decrease in HbA1c. In some patients this was enough to induce diabetes remission. Great news indeed.
The people selling slimfast were not doing randomised control trials, because again they don't care. Simply not what they care about. They care about fancy TV ads and making profit.
NHS does do control trials to actually test, and get actual results. Because they are tasked with actually solving the problem.
Yes they caveat it:
However, this is only one piece in a more complicated jigsaw puzzle. Not everyone who follows the plan will be able to reverse their diabetes. For example, even though the soup and shake diet restricts patient food choice as well as calories, sticking to a liquid only plan can still be difficult for some.
Psychological issues with food, such as emotional eating, aren’t necessarily fully addressed by the programme, so – even for those who’ve followed the plan successfully – once the 12 months have finished, if the patient returns to their usual eating habits then they’ll regain weight and the diabetes could return.
But notice that 12 month followup? That's something you won't get with private companies selling 'slimfast'. But we are getting that with this intervention. Again, different motivations.
In conclusion, be skeptical if you want. It's always good to question. But don't do this reflex conflation with 'slimfast' bs from 40 years ago. Don't shoo away modern, scientific programs that have results, because of some private industry product bs from 40 years ago that never cared in the first place. VERY different things.